Thread Pitch Diameter Calculator
Calculate the pitch diameter, minor diameter, and thread depth for ISO metric and Unified (UNC/UNF) threads. For metric threads, enter the nominal (major) diameter in mm and pitch in mm. For Unified threads, enter the nominal diameter in inches and the threads-per-inch (TPI). Formulas are from ISO 68-1 and ASME B1.1 respectively.
Thread dimension formulas
ISO Metric (external, per ISO 68-1):
Pitch diameter d2 = d - 0.6495 x P
Minor diameter d1 = d - 1.2269 x P
Thread depth h = 0.6134 x P
Unified (external, per ASME B1.1):
Pitch diameter E = D - 0.6495 / n
Minor diameter = D - 1.2990 / n
Thread depth h = 0.6134 / n
Where d/D = nominal (major) diameter, P = pitch (mm), n = threads per inch (TPI). All metric constants derive from the 60-degree thread angle: h = (sqrt(3)/2) x P x (5/8) for full-form threads.
Common metric thread pitch reference
- M6: pitch 1.0 mm
- M8: pitch 1.25 mm
- M10: pitch 1.5 mm (coarse) or 1.25 mm (fine)
- M12: pitch 1.75 mm (coarse)
- M16: pitch 2.0 mm (coarse)
- M20: pitch 2.5 mm (coarse)
Source: ISO 261 Metric screw thread series and ISO 724 for thread dimensions.
Thread pitch diameter: frequently asked questions
What is the pitch diameter of a thread?
The pitch diameter is the theoretical diameter of an imaginary cylinder that passes through the thread profile at the point where the thread groove and thread ridge are of equal width. It is the most important functional diameter for determining thread fit and is used to assess thread gauging and tolerances.
What is the formula for metric thread pitch diameter?
For ISO metric external threads (bolts): Pitch diameter d2 = d - 0.6495 x P, where d = nominal (major) diameter in mm and P = pitch in mm. The constant 0.6495 is derived from the metric thread profile geometry per ISO 68-1.
What is the formula for Unified (UN) thread pitch diameter?
For UNC/UNF external threads: Pitch diameter E = D - 0.6495 / n, where D = nominal diameter (inches) and n = threads per inch. This is equivalent to the metric formula since P = 1/n. Per ASME B1.1.
What is the minor diameter of a thread?
The minor diameter (root diameter) is the smallest diameter of a screw thread, measured at the base of the thread groove. For metric external threads: d1 = d - 1.2269 x P (per ISO 68-1). For internal threads (nuts), the minor diameter is different because the root is at a different height.
What is thread depth and why does it matter?
Thread depth (h) = 0.6134 x P for ISO metric threads. Thread depth determines the engagement length needed for a joint to develop full thread strength. A shorter engagement length reduces holding capacity. Minimum thread engagement length is typically 1.0 to 1.5 times the nominal diameter for steel-to-steel fastening per ASME B1.1.
Official sources
- ISO 68-1: ISO 68-1 General Purpose Screw Threads: Basic Profile.
- ASME B1.1: ASME B1.1 Unified Inch Screw Threads.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.